1. Field of the Invention
The invention relates to a tunable laser light source which involves an uncomplicated arrangement and is remarkable for its high optical stability and specific suppression of the broadband spontaneous emission (ASE: amplified spontaneous emission) and the side modes. One field of application for such a light source is the Raman spectroscopy, among others.
2. Related Art
Tunable laser light sources are known in many variants. FIG. 1 exemplifies a semiconductor laser arrangement. Appropriately modified, this configuration is also used in dye lasers. Essentially, it consists of the laser diode LD, a collimator KO, a diffraction grating GI for dispersing the laser radiation, and a plane mirror SP rotatable in the dispersion direction of the grating. The laser radiation forming an essentially parallel beam after the collimator KO is diffracted at the grating GI and reaches the rotatable mirror SP. Only those laser wavelengths diffracted in such a fashion as to strike the mirror SP substantially vertically return on the same path with sufficient accuracy and are imaged on the active laser facet, thereby generating an optical back-coupling. Thus, the wavelength range coupled back and hence, the emission major wavelength of the arrangement may be selected by merely rotating the mirror SP in the dispersion direction of the grating GI.
The usable laser radiation, on the other hand, is decoupled by the zero order diffraction of the grating, for which purpose the radiation is focussed, e.g., by a lens O in an optical waveguide LWL. Independently of the wavelength adjustment, the usable radiation always appears at the same position.
On the one hand, these arrangements which are widely used in the form of the above or a similar type, are disadvantageous in that they are highly sensitive even to minor maladjustment. For example, because the optically effective facet of a semiconductor laser is very small, the configuration is required to have particular stability with respect to inclinations of the optical path vertically to the dispersion direction. This involves the bearing of the rotatable mirror, the stability of grating support, laser and collimator. In such configurations, three degrees of freedom must be controlled, only one of which being imperatively required, namely, the rotation of the mirror for wavelength tuning. The displacement of the laser vertically to the dispersion direction of the dispersing element, as well as the displacement of the laser chip along the optical axis, which is required for focussing, must be held in optimum position without permitting any of the starting parameters to be changed. One aspect that complicates the problem is that these two independently adjustable coordinates cannot be optimized separately, so that naturally, there is only one optimum position in this two-dimensional method of adjusting. Frequently, a special procedure is used for this object.
Another drawback of such arrangements is that rather than spectrally purified radiation, a fraction of the entire radiation mix in the resonator, including the spontaneous emission and more or less strong side modes, is taken from the resonator as usable radiation. In addition, since decoupling of the radiation takes place through a path different from that of the back-coupling, barely avoidable reflections from the external arrangement wherein the laser radiation is used may have massive influence on the radiation in the resonator, unless additional means are employed, such back-coupling normally not taking place in a wavelength-selective fashion. Inherently, this is the case if an optical image of the laser facet is present on an at least partially reflecting, not necessarily mirroring surface (e.g., optical waveguide, receiver surfaces), because such arrays act as retroreflectors.
Solutions are known which either allow to obtain a radiation which is spectrally pure to a large extent, and those which increase the adjusting tolerance of such a laser resonator by using special measures, thus permitting to build up a comparably robust apparatus. Until now, however, an arrangement that would combine both of these features is not known.
In this context, the state of the art in obtaining spectrally pure radiation is represented by the DE-AS 29 18 863. In this invention, the radiation that has already left the resonator is essentially directed into a device for its spectral purification. Here, in particular, the dispersing element used in the wavelength selection of the laser is also used by said device under at least largely equal conditions, thereby achieving that it is always the appropriately filtered radiation which leaves the arrangement, regardless of the laser wavelength adjustment. However, the one drawback still remains that substantially, it is only the dispersing element which is used doubly and thus, various additional components are required for redirecting the radiation into the filter mechanism and for the latter itself. In a variant of the above-mentioned DE-AS 29 18 863, an arrangement is described where part of the filtered radiation remains in the resonator or is returned into same. However, apart from the dispersing element, both laser and filter device are present therein as well. In addition, a substantial part of non-filtered radiation leaves the resonator through a separating mirror which is necessarily present, so that this part is lost.
The DE OS 42 16 001 A1 is also directed to obtaining spectrally purified radiation. Therein, the entire radiation in the resonator and the spectrally purified fraction pass through the resonator at different angles, thus enabling their separation. However, several components are operated under grazing incidence, thereby impairing the applicability of the above invention. In addition, the spectrally purified radiation as well passes through the laser medium one more time, thereby rendering the spectral purity doubtful again.
Essentially, the state of the art for increasing the alignment tolerance in lasers having external resonators is governed by two solutions: The first solution is described in P. Zorabedian and W. R. Trutna, Jr.: Interference-filter-tuned, alignment-stabilized, semiconductor external-cavity laser, OPTICS LETTERS, Vol. 13, No. 10, pp 826-828. A cat's eye retroreflector (converging lens with a mirror in its focal plane) is used in the alignment-tolerant back-coupling of laser radiation. In the parallel optical path inside the resonator, there is an interference filter as selective element. For tuning the laser wavelength, said filter is supported rotatably. The decoupling of the usable radiation takes place from that facet of the laser chip which is on the side facing away from the external resonator.
However, this arrangement involves the drawbacks that the broadband spontaneous emission and the side modes cannot be separated without using substantial additional means, and there are limitations resulting from the properties of an interference filter.
The EP 0,525,752 A1 includes another way to build up an alignment-stable laser having an external resonator. Therein as well, a cat's eye retroreflector is used in principle, but its effect is limited to one coordinate. By means of a suitable combination of prisms and a cylinder lens for beam shaping, and by using a diffraction grating as reflector, it is found that the laser facet is imaged on the grating only vertically to the dispersion direction. In dispersion direction, however, the radiation beam striking the grating is mainly parallel and relatively broad. In this way it has been achieved that the grating can be used for tuning the laser wavelength without restriction and, on the other hand, the arrangement is largely tolerant with respect to inclination of the grating vertically to the dispersion direction. Without additional means, however, this arrangement neither permits separating the broadband spontaneous emission and side modes from the usable fraction of radiation.